The present invention relates to a luminance/chrominance separating filter for separating luminance and chrominance signals from, for example, an NTSC composite color television signal.
FIG. 43 shows an example of a conventional luminance/chrominance separating filter (YC separating filter) with an NTSC composite color television signal applied to the input terminal 11. An A/D converter 12 converts the analog composite color television signal input via the input terminal 11 into a digital signal. The output signal of the A/D converter 12 is applied to a first 1-line delay circuit 13.
The conventional system further comprises a second 1-line delay circuit 14, a compensating delay circuit 19, a vertical chrominance (V-C) extraction filter 15, a horizontal chrominance (H-C) extraction filter 16, a horizontal and vertical chrominance (HV-C) extraction filter 17, a picture non-correlation decision circuit 18, compensating delay circuits 19-22, a selector 23 provided with an output terminal 26, and a subtractor 27 provided with an output terminal 28.
FIG. 11 shows an example of the picture non-correlation decision circuit 18 in FIG. 43. As illustrated, the picture non-correlation decision circuit 18 comprises a horizontal chrominance non-correlation (H-C) energy extractor 29, a horizontal luminance (H-Y) non-correlation energy extractor 30, a vertical chrominance (V-C) non-correlation energy extractor 31, a vertical luminance (V-Y) non-correlation energy extractor 32, maximum value circuits (MAXs) 33-36, comparators 37-39, a decision circuit 40, multipliers 71, 72, 73a, 73b, 74a, 74b, 77, 78, 79a, 79b and 80, and delay circuits 86-89.
The output signal 101 from the A/D converter 12 in FIG. 43 is applied to the horizontal luminance non-correlation energy extractor 30, the vertical chrominance non-correlation energy extractor 31 and the vertical luminance non-correlation energy extractor 32.
In the foregoing, "non-correlation energy" refers to the signal high frequency component arising, depending on the degree of signal variation. As the signal variation is increased, the sharper the high frequency component in the direction of the variation is increased. Thus, the "non-correlation energy" represents the signal high frequency component energy, which can be obtained by extracting the required band (high-frequency band) component through a two dimensional (or one dimensional) filtering process (energy extraction). The required band can be expressed on a two dimensional frequency plane, shown for example in FIG. 44 to FIG. 47.
The output signal 102 of the first 1-line delay circuit 13 is applied to the horizontal chrominance non-correlation energy extractor 29, the horizontal luminance non-correlation energy extractor 30 and the vertical luminance non-correlation energy extractor 32.
The output signal 103 of the second 1-line delay circuit 14 is applied to the horizontal luminance non-correlation energy extractor 30, the vertical chrominance non-correlation energy extractor 31 and the vertical luminance non-correlation energy extractor 32.
The output signal DCH of the horizontal chrominance non-correlation energy extractor 29 is branched in three routes. In the first route, the signal is multiplied by a coefficient b at the multiplier 72 and the resultant signal is sent to the maximum value circuit 33. In the second route, the signal is multiplied by a coefficient f1 at the multiplier 74a and the resultant signal is sent to the maximum value circuit 34. In the third route, the signal is multiplied by a coefficient f2 at the multiplier 74b and the resultant signal is sent to the maximum value circuit 35.
The output signal DYH of the horizontal luminance non-correlation energy extractor 30 is branched in three routes. In the first route, the signal is multiplied by a coefficient a at the multiplier 71 and the resultant signal is sent to the maximum value circuit 34. In the second route, the signal is multiplied by a coefficient e1 at the multiplier 73a and the resultant signal is sent to the maximum value circuit 34. In the third route, the signal is multiplied by a coefficient e2 at the multiplier 73b and the resultant signal is sent to the maximum value circuit 35.
The output signal DCV of the vertical chrominance non-correlation energy extractor 31 is branched in two routes. In the first route, the signal is sent to the comparator 38. In the second route, the signal is multiplied by a coefficient d at the multiplier 78 and the resultant signal is sent to the maximum value circuit 36.
The output signal DYV of the vertical luminance non-correlation energy extractor 32 is branched in two routes. In the first route, the signal is sent to the comparator 39. In the second route, the signal is multiplied by a coefficient c at the multiplier 77 and the resultant signal is sent to the maximum value circuit 36.
The output signal of the maximum value circuit 33 is applied as a first horizontal non-correlation energy DH1 to the comparator 37.
The output signal of the maximum value circuit 34, as a second horizontal non-correlation energy DH21, is multiplied by a coefficient m1 at the multiplier 79a, and is then sent to the comparator 38.
The output signal of the maximum value circuit 35, as a third horizontal non-correlation energy DH22, is multiplied by a coefficient m2 at the multiplier 79b, and is then sent to the comparator 39.
The output of the maximum value circuit 36, as a vertical non-correlation energy DV, is multiplied by a coefficient n, and is then sent to the comparator 37.
The comparator 37 compares the first horizontal non-correlation energy DH1 and the product n.multidot.DV obtained by multiplying the vertical non-correlation energy DV by the coefficient n, and produces a high level output signal 116 when DH1.gtoreq.n.multidot.DV, and a low level output signal 116 at other times.
The comparator 38 compares the vertical chrominance non-correlation energy DCV and the product m1.multidot.DH21 obtained by multiplying the second horizontal non-correlation energy DH21 by the coefficient m1, and produces a high level output signal 117 when DCV.gtoreq.m1.multidot.DH21, and a low level output signal 117 at other times.
The comparator 39 compares the vertical luminance non-correlation energy DYV and the product m2.multidot.DH22 obtained by multiplying the third horizontal non-correlation energy DH22 by the coefficient m2, and produces a high level output signal 118 when DYV.gtoreq.m2.multidot.DH22, and a low level output signal 118 at other times.
The output signal 116 of the comparator 37 is applied to the delay circuit 86, and the output signal 117 of the comparator 38 is applied to the delay circuit 87.
The output signal 118 of the comparator 39 is applied to the delay circuit 88 and the AND circuit 90, and the output signal 121 of the delay circuit 88 is applied to the delay circuit 89 and the AND circuit 90.
The output signal 122 of the delay circuit 89 is applied to the AND circuit 90. The output signal 119 of the delay circuit 86, the output signal 120 of the delay circuit 87 and the output signal 124 of the AND circuit are applied to the decision circuit 40. The output signal 110 of the decision circuit 40 is then sent out as the output of the picture non-correlation decision circuit 18.
FIG. 48 shows an example of the decision circuit 40 in FIG. 11. The circuit comprises AND circuits 41 and 42, a NOT circuit 43 and a NOR circuit 44. The output signal 119 of the delay circuit 86 is applied to one input of the AND circuit 42 and to the input of the NOT circuit 43. The respective output signals 120 and 123 of the delay circuit 87 and AND circuit 90 are applied to the NOR circuit 44.
The output signal of the NOR circuit 44 is applied to the other input of the AND circuit 42 and to one input of the AND circuit 41. The outpost signal of the NOT circuit 43 is applied to the other input of the AND circuit 41.
The outputs of the AND circuits 41 and 42 form the output signal 110 of the decision circuit 40.
FIG. 49 shows an example of the horizontal chrominance non-correlation energy extractor 29 in FIG. 11. The circuit comprises a delay circuit 45 having a delay equivalent to one period (1/fsc) of the color subcarrier having a frequency fsc, a subtractor 46 and an absolute value circuit (ABS) 47.
The output signal 102 of the first 1-line delay circuit 13 is applied to the delay circuit 45 and one input of the subtractor 46. The output signal of the delay circuit 45 is applied to the other input of the subtractor 46. The output signal of the subtractor 46 is applied to the absolute value circuit 47.
The output of this absolute value circuit 47 forms the horizontal chrominance non-correlation energy.
FIG. 50 shows an example of the horizontal luminance non-correlation energy extractor 30 used in FIG. 11. The circuit comprises a vertical direction lowpass filter (LPF) 48, delay circuits 49 and 50 each having a delay equivalent to 1/2 the period (1/2fsc) of the color subcarrier, subtractors 51 and 52, absolute value circuits 53 and 54, and a maximum value circuit 55.
The respective output signals 101, 102 and 103 of the A/D converter 12, the first 1-line delay circuit 13 and the second 1-line delay circuit 14 are applied to the vertical direction lowpass filter 48.
The output signal of the vertical direction lowpass filter 48 is applied to the delay circuit 49 and one input of the subtractor 51. The output signal of the delay circuit 49 is applied to the delay circuit 50, the other input of the subtractor 51 and one input of the subtractor 52. The output signal of the delay circuit 50 is applied to the other input of the subtractor 52.
The output signal of the subtractor 51 is applied to the absolute value circuit 53 and the output signal of the absolute value circuit 53 is applied to the maximum value circuit 55. The output signal of the subtractor 52 is applied to the absolute value circuit 54 and the output signal of the absolute value circuit 54 is applied to the maximum value circuit 55.
The output signal of the maximum value circuit 55 forms the output DYH of the horizontal luminance non-correlation energy extractor 30.
FIG. 51 shows an example of the vertical chrominance non-correlation energy extractor 31 in FIG. 11. The circuit comprises horizontal bandpass filters (BPFs) 56 and 57, a subtractor 58 and an absolute value circuit 59.
The output signal 101 of the A/D converter 12 is applied to the horizontal bandpass filter 56. The output signal 103 of the second 1-line delay circuit 14 is applied to the horizontal bandpass filter 57. The output signal of the horizontal bandpass filter 56 is applied to one input of the subtracter 58, while the output signal of the horizontal bandpass filter 57 is applied to the other input of the subtracter 58. The output signal of the subtracter 58 is applied to the absolute value circuit 59. The output signal of the absolute value circuit 59 forms the output DCV of the vertical chrominance non-correlation energy extractor 31.
FIG. 52 shows an example of the vertical luminance non-correlation energy extractor 32 used in FIG. 11. The circuit comprises horizontal lowpass filters 60, 61 and 62, subtracters 63 and 64, absolute value circuits 65 and 66, and a maximum value circuit 67.
The output signal 101 of the A/D converter 12 is applied to the horizontal lowpass filter 60. The output signal 102 of the first 1-line delay circuit 18 is applied to the horizontal lowpass filter 61. The output signal 103 of the second 1-line delay circuit 14 is applied to the horizontal lowpass filter 62.
The output signal of the horizontal lowpass filter 60 is applied to one input of the subtractor 63. The output signal of the horizontal lowpass filter 61 is applied to the other input of the subtractor 63 and to one input of the subtractor 64. The output signal of the horizontal lowpass filter 62 is applied to the other input of the subtractor 64.
The output signal of the subtractor 63 is applied to the absolute value circuit 65 and the output signal of the subtractor 64 is applied to the absolute value circuit 66.
The outputs off the absolute value circuits 65 and 66 are applied to the maximum value circuit 67. The output signal of the absolute value circuit 67 forms the output DYV of the vertical luminance non-correlation energy extractor 32.
Following is a description of the principle and operation of the conventional YC separating filter indicated in FIG. 43, FIG. 11, and FIG. 48 to FIG. 52.
When the horizontal frequency .mu. and vertical frequency .nu. axes are plotted on a two dimensional plane, the NTSC composite color television signal distribution appears as shown in FIG. 44. This signal distribution takes different forms according to the two dimensional correlation of the picture. For example, if the correlation is weak in the vertical direction, but strong in the horizontal, the signal distribution appears as indicated in FIG. 45. In this case, the chrominance signal can be extracted using a horizontal chrominance extraction filter having a passband as indicated by the shaded rectangles in the figure.
If the correlation is strong in the vertical direction, but weak in the horizontal, the signal distribution appears as indicated in FIG. 46. In this case, the chrominance signal can be extracted using a vertical chrominance extraction filter having a passband as indicated by the shaded rectangles in the figure.
When the correlation is strong in both directions, the signal distribution appears as indicated in FIG. 47. In this case, the chrominance signal can be extracted by using a horizontal and vertical chrominance extraction filter having a passband as indicated by the shaded rectangles in the figure.
As indicated in FIG. 43, in the case of this YC separating filter, selector 23, selects in accordance with the output of the picture non-correlation decision circuit 18, one of the output from the vertical chrominance extraction filter 15, the horizontal chrominance extraction filter 16, and the horizontal and vertical chrominance extraction filter 17.
It will be seen from FIG. 45 that the condition on which the horizontal chrominance extraction filter 16 is selected is that there is much chrominance signal vertical high frequency component (there is much color component in the shaded rectangles) and there is little luminance signal horizontal high frequency component (there is little color component in the shaded rectangles).
It will be seen from FIG. 46 that the condition on which the vertical chrominance extraction filter 15 is selected is that there is much chrominance signal horizontal high frequency component (there is much color component in the shaded rectangles) and there is little luminance signal vertical high frequency component (there is little luminance component in the shaded rectangles).
If neither of the conditions for selecting either of the above horizontal or vertical chrominance signal extraction filters is met, the horizontal and vertical chrominance extraction filter 17 is selected.
Following is a description of the process from derivation of these conditions to the filter selection.
When an NTSC composite color television signal is applied via the input terminal 11, the A/D converter 12 samples this composite color television signal at a sampling frequency fs=4.multidot.fsc (fsc is the color subcarrier frequency). The sampled composite color television signal forms a two dimensional arrangement on the screen as indicated in FIG. 53. Since fsc=(455/2)fH, the phase of the chrominance signal C reverses 180 degrees every line and 4 samples per period are taken.
In the figure, Y denotes the luminance signal, C1 and C2 denote chrominance signals having a 180.degree. phase difference, blank circles are Y+C1, shaded circles are Y-C1, blank triangles are Y+C2 and shaded triangles are Y-C2. By passing this composite color television signal through the first and second 1-line delay circuits 13 and 14, the sample value at a specific sampling point and two reference sampling points respectively one line above and one line below the specific sample tag point on the screen are simultaneously extracted. Here the term "specific sampling point" means the sampling point for which the signal processing is made, and the term "reference sampling points" means sampling points which are situated in the neighborhood of the specific sampling point when the sampling points are arranged on a two-dimensional plane corresponding to a display screen used for the display of the picture.
In other words, at the time point the composite color television signal (sample value) S(m, n) at the position of coordinate (m, n) appears at the output 102 of the first 1-line delay circuit 13, the signal (m, n-1) appears at the output 103 of the second 1-line delay circuit 13 and the signal (m, n+1) appears at the output 101 of the A/D converter 12.
The signal 102 is applied to the horizontal chrominance extraction filter 16. This signal 102 and the other two signals 101 and 103 are respectively applied to the vertical chrominance extraction filter 15, the horizontal and vertical chrominance extraction filter 17, and the picture non-correlation decision circuit 18.
Using Z conversion, 1-sample delay and 1-line delay can be expressed by Z.sup.-1 and Z.sup.-L, respectively. In this case, EQU Z.sup.-1 =exp(-j2.pi.f/4fsc)
Also, since fsc=(455/2), L=910. For example, in this case, the transfer function of the vertical chrominance extraction filter 15 is expressed as follows. EQU Cv(Z)=(-1/4)(1-Z.sup.-L).sup.2
The transfer function of the horizontal chrominance extraction filter 16 is expressed as follows. EQU Ch(Z)=(-1/4)(1-Z.sup.-2).sup.2
The transfer function of the horizontal and vertical chrominance extraction filter 17 is expressed as follows. EQU Chv(Z)=(-1/4)(1-Z.sup.-2).sup.2 .multidot.(-1/4)(1-Z.sup.-L).sup.2
The output signal 106 of the vertical chrominance extraction filter 15 is supplied as the output signal 107 of the compensating delay circuit 20 to the selector 23. The output signal 108 of the horizontal chrominance extraction filter 16 is supplied as the output signal 109 of the compensating delay circuit 21 to the selector 23. The output signal 110 of the horizontal and vertical chrominance extraction filter 17 is supplied as the output signal 111 of the compensating delay circuit 22 applied to the selector 23.
The vertical and horizontal picture non-correlation of a specific sampling point is detected and the selector 23 functions in the following manner.
When the horizontal non-correlation is particularly strong, the output signal 107 of the compensating delay circuit 20 to which the output signal 106 of the vertical chrominance extraction filter 15 is supplied is selected. When the vertical non-correlation is particularly strong, the output signal 109 of the compensating delay circuit 21 to which the output signal 108 of the horizontal chrominance extraction filter 16 is selected. At other times, the output signal 111 of the compensating delay circuit 22 to which the output signal 110 of the horizontal and vertical chrominance extraction filter 17 is supplied is selected.
The detection of the picture non-correlation and control over the selector 23 are performed by the picture non-correlation decision circuit 18. This picture non-correlation decision circuit 18 operates in the following manner to control the selector.
The horizontal chrominance non-correlation energy represented DCH(Z), the horizontal luminance non-correlation energy DYH(Z), the vertical chrominance non-correlation energy DCV(Z) and the vertical luminance non-correlation energy DYV(Z) can be expressed as follows. EQU DCH(Z)=.vertline.1-Z.sup.-4 .vertline. EQU DYH(Z)=max{.vertline.(1/4).multidot.(1+Z.sup.-L).sup.2 .multidot.(1-Z.sup.-2).vertline., .vertline.(1/4).multidot.(1+Z.sup.-L).sup.2).multidot.(Z.sup.-2 -Z.sup.-4).vertline.} EQU DCV(Z)=.vertline.(-1/4).multidot.(1-Z.sup.-2).sup.2 .multidot.(1-Z.sup.-2L).vertline. EQU DYV=max {.vertline.(1/4).multidot.(1+Z.sup.-2).sup.2 (1-Z.sup.-L).vertline., .vertline.(1/4).multidot.(1+Z.sup.-2).sup.2).multi dot.(Z.sup.-L -Z.sup.-2L).vertline.}
The first horizontal non-correlation energy DH1, the second horizontal non-correlation energy DH21, the third horizontal non-correlation energy DH22 and the vertical non-correlation energy DV can be expressed as follows. EQU DH1=max(a.multidot.DYH, b.multidot.DCH) EQU DH21=max(e1.multidot.DYH, f1.multidot.DCH) EQU DH22=max(e2.multidot.DYH, f2.multidot.DCH) EQU DV=max(c.multidot.DYV, d.multidot.DCV)
At the comparator 37, DH1 and n.multidot.DV are compared. If EQU DH1.gtoreq.n.multidot.DV
the horizontal non-correlation is interpreted as strong, and a "1" signal 116 is sent to the delay circuit 86. If EQU DH1&lt;n.multidot.DV
the horizontal non-correlation is interpreted as weak, and a "0" signal 116 is sent to the delay circuit 86. This signal 116 is delayed 1/2fsc by the delay circuit 86 and is sent as the signal 119 to the decision circuit 40.
At the comparator 88, DCV and m1.multidot.DH21 are compared. If EQU DCV.gtoreq.m1.multidot.DH21
the vertical non-correlation is interpreted as strong, and a "1" signal 117 is sent to the delay circuit 87. If EQU DCV&lt;m1.multidot.DH21
the horizontal non-correlation is interpreted as weak, and a "0" signal 117 is sent to the delay circuit 87. The signal 117 delayed 1/2fsc by the delay circuit 87 is sent as a signal 120 to the decision circuit 40.
At the comparator 39, DYV and m2.multidot.DH22 are compared. If EQU DYV.gtoreq.m2.multidot.DH22
the vertical non-correlation is interpreted as strong, and a "1" signal 118 is sent to the delay circuit 88 and the AND circuit 90. If EQU DYV&lt;m2.multidot.DH22
the vertical non-correlation is interpreted as weak, and a "0" signal 118 is sent to the delay circuit 88 and the AND circuit 90.
The output signal 121 of the delay circuit 88 is sent to the delay circuit 89 and the AND circuit 90. The output signal 122 of the delay circuit 89 is sent to the AND circuit 90. The output signal 123 of the AND circuit 90 is applied to the decision circuit 40.
According to the results or the above correlation detection, the decision circuit 40 controls the selector 23 in the following manner. That is, the relationship between the input signals 119, 120 and 123 of the decision circuit 40 and the selection of the chrominance output signal 107, 109 or 111 at the selector 23 is as shown in Table 1.
TABLE 1 __________________________________________________________________________ Output 1120 of Chrominance Output 119 Output 120 Output 123 Decision Circuit 40 Output 113 of Delay of Delay of Delay Output 110a Output 110b Selected by Circuit 86 Circuit 87 Circuit 90 of AND 41 of AND 42 Selector 23 __________________________________________________________________________ 0 0 0 1 0 111 0 0 1 0 0 109 0 1 0 0 0 109 0 1 1 0 0 109 1 0 0 0 1 107 1 0 1 0 0 109 1 1 0 0 0 109 1 1 1 0 0 109 __________________________________________________________________________
When the output signal 110a of the AND circuit 41 and the output signal 110b of the AND circuit 42 are both "0", the selector 23 selects the output: signal 109 of the compensating delay circuit 21. When the output signal 110a of the AND circuit 41 is "0" and the output signal 110b of the AND circuit 42 is "1", the selector 23 selects the output signal 107 of the compensating delay circuit 20. When the output signal 110a off the AND circuit 41 is "1" and the output signal 110b of the AND circuit 42 is "0", the selector 23 selects the output signal 111 of the compensating delay circuit 22.
Consequently, in this example, the overall C(z) response of the filter for extracting the chrominance signal to be output at the output terminal 26 is switched as follows according to the presence or absence of correlation. When the vertical non-correlation is strong: EQU C(Z)=Ch(Z)
When the horizontal non-correlation is strong: EQU C(Z)=Cv(Z)
If neither of these conditions are met: EQU C(Z)=Chv(Z)
In the conventional YC separating filter described above, the luminance signal Y and the chrominance signal C are separated by adaptively selecting one of the horizontal filter, the vertical direction filter, and the horizontal and vertical direction filter. As a result, in areas where the picture horizontal or vertical direction luminance or chrominance signal variation is sharp, e.g., in areas at the boundary between different color regions, such as at an edge of a color bar signal, picture quality impairment such as dot crawl (due to leakage of the chrominance signal component into the luminance signal) does not occur. However, the conventional YC separating filter described above is associated with the problems of considerable cross color (due to leakage of high frequency luminance components into chrominance signal areas), particularly at the areas where the picture includes inclined fine stripes or gratings, and the diagonal resolution is insufficiently high.